This invention relates to a color liquid crystal display device and a method for producing a color filter substrate. More particularly, it relates to a color liquid crystal display device having superior display quality and high reliability, and a method for producing a color filter substrate employed therefor.
Recently, attention is drawn to a color liquid crystal display device as a flat panel display device. In general, a color liquid crystal display device is of such a construction in which a liquid crystal layer is sandwiched between a color filter substrate having a black matrix and colored layers of plural colors, usually red (R), green (G) and blue (B) colors, and a counterelectrode substrate. A color image is produced by controlling light transmittance of the portions of the liquid crystal layer in register with respective pixels of the colored layers R, G and B. Thus, the color filter substrate is indispensable to the color liquid crystal display device and represents a crucial member influencing the display quality itself of the color liquid crystal display device.
Heretofore, the black matrix of the color filter substrate is formed by photolithographically patterning a chromium or chromium oxide layer formed by sputtering or vapor deposition. On the other hand, the colored layers of the color filter substrate are formed by a dyeing method comprising dyeing a pattern obtained by coating dyeing materials and subjecting to light exposure via a photomask followed by development, a pigment dispersion method comprising previously dispersing coloring pigments in a photosensitive resist and subsequently exposing to light the resulting dispersant via a photomask followed by development, or a printing method comprising printing respective colors using printing inks.
However, the color filter substrate comprised of the black matrix and the colored layers, is complicated in the production process and high in production cost, while being unable to fully satisfy the properties required of the color filter substrate. Thus, a demand has been raised for a color filter substrate which is less expensive and higher in performance.
For meeting such demand, a black matrix having colorants, pigments, carbon black or fine metal particles dispersed in a resin is attracting attention insofar as the material aspect is concerned. Such black matrix is fabricated by a black photoresist method comprising dispersing colorants, pigments, carbon black or fine metal particles in a photosensitive resin and subsequently patterning the resulting dispersant by a photolithographic technique. The black matrix may also be fabricated by an electroless plating method comprising precipitating fine metal particles into a resin, or an electrodeposition method comprising forming an electrodeposition coating containing colorants, pigments, carbon black or fine metal particles and carrying out electrodeposition.
In addition, an electrodeposition method is excellent in utilization efficiency of coating materials and production capability and the time for forming the black matrix or the colored layers is as short as about several ten seconds. Thus, the electrodeposition method attracts attention insofar as the production process is concerned.
With such electrodeposition method, a transparent electrically conductive layer is formed on a transparent substrate, and a positive photoresist layer is formed on the transparent electrically conductive layer. The resulting assembly is exposed to light via a mask having a pre-set pattern, and the light exposed portion is dissolved and removed for laying-open a pre-set portion of the transparent electrically conductive layer. The transparent substrate is then immersed in an electrodeposition solution of a pre-set color, and electrical current is supplied from the transparent electrically conductive layer on the transparent substrate for electrodeposition. The black matrix and the colored layers are sequentially formed by repetition of the electrodepositing operations. After the black matrix and the colored layers are formed in this manner on the substrate, the entire surface of the substrate is irradiated with UV rays for removing the positive photoresist layer. A transparent electrically conductive layer for driving the liquid crystal layer is then formed on the black matrix and the colored layers. A polyimide alignment layer is then applied on the transparent electrically conductive layer. The color filter substrate is produced after baking and rubbing by way of alignment processing.
In producing a color filter substrate and a counterelectrode substrate employed for a color liquid crystal display device, a plurality of color filters, that is, not less than two filters for large-sized office equipments and six to twelve color filters for small-sized 4 to 6 inch class handy television receivers, are placed thereon for improving the production efficiency. The color filter substrate having plural color filters thereon and the counterelectrode substrate, are bonded together to form a large number of cells and are subsequently cut by a glass scriber into unit cells. These cells are processed with liquid crystal injection, sealing of the injection ports, boding of polarizing plates, formation of driving ICs and peripheral circuitry and provision of backlights and a casing to complete a color liquid crystal display device.
However, with the color liquid crystal display device employing a color filter substrate having the above-described resin-based black matrix and colored layers, it is difficult to set the thickness of the black matrix to not larger than 1.5 .mu.m in consideration of light-shielding characteristics proper to the black matrix, thus raising various problems due to step differences at the boundary portions between the black matrix forming region and the remaining region.
FIG. 6 shows, in a partial cross-sectional view, the vicinity of an edge portion of a conventional color liquid crystal display device 101 employing a color filter substrate fabricated using the electrodeposition method. Referring to FIG. 6, the conventional color liquid crystal display device 101 has a color filter substrate 102 and a counterelectrode substrate 103 facing each other with interposition of a pre-set gap. The peripheral portions of the two substrates 102, 103 are sealed with a sealing material 104 and a liquid crystal layer 105 is formed between the two substrates 102, 103. Polarizing plates 106, 106 are provided on the outer sides of the color filter substrate 102 and the counterelectrode substrate 103. On a transparent substrate 111 of the color filter substrate 102 are formed a black matrix 116 and colored layers 117 in a pre-set pattern via a transparent electrically conductive layer 112. A transparent electrode 118 for driving the liquid crystal and an alignment layer 119 are further formed thereon. The counterelectrode substrate 103 has on its transparent substrate 121 a transparent electrode 122 for driving the liquid crystal and thin film transistors (TFT) 123, and an alignment layer 124 is formed for overlying the transparent electrode 122. The counterelectrode substrate 103 carries gate bus lines, not shown, for turning the thin film transistors (TFT) 123 on and off, source bus lines, not shown, for furnishing video signals, and voltage supply lines to the color filter electrode from the corners of the color filter substrate. The lead wires 125 are formed of metals, such as Al, collectively formed in the course of the production process of the thin film transistors (TFT) 123, and are connected to the transparent electrode 122 and to an electrical connection line 132 from the external driving IC 131.
With the above-described conventional color liquid crystal display device 101, the sealing material 104 for bonding the color filter substrate 102 and the counterelectrode substrate 103 to each other for forming plural cells is configured to abut against both the peripheral portions of the black matrix 116 and the transparent electrically conductive layer 112. However, the step difference of not less than 1.5 .mu.m is produced between the black matrix forming region and the black matrix non-forming region, that is the exposed portion of the transparent electrically conductive layer 112, as described above. Due to this step difference, the sealing material cannot be applied with high precision, resulting in variation in the coating quantity of the sealing material and insufficient hermetic sealing of the cells and possibly leading to operational troubles of the liquid crystal under high temperature and high humidity conditions. On the other hand, when the color filter substrate 102 and the counterelectrode substrate 103 are pressed against and bonded to each other, a spacer for controlling the gap is incorporated into the sealing material for coping with the step difference. However, the sealing material is then liable to flow non-uniformly, thus producing sealing troubles and leading to failure in producing high cell gap precision.